The present invention relates to photomasks, and more particularly, to a damascene structured photomask formed for use with extreme ultraviolet (EUV) light illuminating radiation.
Photolithography is a common step used in the manufacture of integrated circuits and is typically carried out in a tool known as a xe2x80x9cstepperxe2x80x9d. In photolithography, a silicon wafer substrate having a layer of film to be patterned is covered with a layer of photoresist. The wafer is then placed within a stepper onto a stage. A photomask is placed above and over the wafer. The photomask (also known as a reticle) contains the pattern that is to be replicated onto the wafer.
In the case of a transmissive photomask, the mask pattern is created by transmissive portions and absorbing portions arranged in the pattern on the mask. A selected wavelength, for example, 248 nanometers (nm), of irradiating radiation is shined through the mask. The transmissive portions of the mask, which are transparent to the selective wavelength, allow the light to pass through the mask. The absorbing portions, which are opaque to and absorb the selected wavelength, block the transmission. The pattern on the mask is thereby replicated onto the photoresist on the device wafer.
In another type of photomask, known as a reflective mask, the photomask surface contains reflective portions and absorbing portions. When light of a selected wavelength is applied to the photomask, the light is reflected off the reflecting portions. The reflected image from the mask usually is further reflected off of a mirror or lens system, then onto the wafer. Once exposed, the photoresist on the wafer is developed by rinsing in a solution that dissolves either exposed or unexposed portions of the photoresist, depending upon positive or negative tone of the photoresist, to create a pattern in the photoresist that matches the pattern of the photomask.
As device integration increases, the dimensions of features in the integrated circuit devices also must shrink. Therefore, the illuminating radiation used in photolithography must have shorter and shorter wavelengths to pattern successfully in shrinking dimensions. Patterning using 193 nm and 157 nm as wavelengths are all currently being developed. These wavelengths are generically known as the deep UV range (193 nm) and vacuum UV range (157 nm). However, EUV radiation is strongly absorbed generally by condensed matter, such as quartz. Thus, a reflective photomask is commonly used for EUVL.
Generally, the reflective mask consists of a multilayer stack of pairs of molybdenum and silicon thin films. The multilayer stack will reflect EUV radiation. Formed atop of the multilayer stack is a patterned absorptive metal layer. The patterned absorptive metal layer is patterned and etched from a blanket metal layer that is deposited onto the multilayer stack. This type of reflective mask is known as a subtractive metal reflective mask.
Another type of reflective mask is known as a damascene reflective mask. In this type of mask, trenches are formed in a silicon base layer that is deposited atop of the multilayer. The trenches are then filled with an absorptive metal layer. One such damascene reflective mask is described in detail in U.S. Pat. No. 5,935,733 to Scott et al. and assigned to the assignee of the present invention.
Specifically, turning to FIG. 1, a prior art photomask 101 is shown that includes a multilayer stack 103, a silicon base layer 105, a metal absorber layer 107, and a cap silicon layer 109. The multilayer stack 103 comprises alternating thin film layers of molybdenum (Mo) and silicon (Si). Typically, the multilayer stack 103 consists of 40 pairs of Mo/Si thin films, each pair of thin films approximately 7 nm in thickness. Next, the amorphous silicon base layer 105 is deposited onto the multilayer stack 103. Trenches are etched into the silicon base layer 105 and an absorbing metal 107 is deposited into the trenches. Finally, a silicon capping layer 109 is deposited to protect the photomask from damage.
In operation, incident EUV light 111 is reflected by the multilayer stack 103. Incident light on to the absorbing metal layer 107 is absorbed. This prior art photomask has several disadvantages. First, the silicon layer 105 tends to attenuate the amount of EUV light 111 that is reflected by the multilayer stack 103. For a 70 nm thickness of silicon, the attenuation is on the order of 22%. This attenuation will eventually lower the throughput of the stepper machine. In other words, a photoresist layer that may ordinarily take five seconds to expose, because of the attenuation in the silicon layer 105, may require six to seven seconds to expose.
A second disadvantage can be seen in FIG. 1 and is referred to as the shadowing effect. Because in EUVL, the incident radiation comes at an angle from normal due to the nature of a reflective mask, the combination of oblique illumination with a non-zero height metal layer 107, a shadowing effect exists which needs to be corrected by adjusting the size of the photomask features. Typically, the photomask is biased toward a smaller dimension in order to compensate for the shadowing effect. As EUVL technology extends to smaller design rules, the biasing requirement may place a limitation on EUVL mask fabrication.
Finally, for a subtractive metal reflective mask, while the absorptive metal layer is a conductive layer, its passivating silicon oxide layer is non-conducting. The non-conductive photomask may cause pinhole defects during mask transfer or handling processes due to charging. Any charge build up during handling and exposure can attract charged particles that are very difficult to remove.